1. Technical Field
Exemplary aspects of the present disclosure generally relates to a transfer device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to a transfer device to transfer an unfixed toner image onto a recording medium by applying a transfer bias, and an image forming apparatus including the transfer device.
2. Description of the Related Art
In recent years, a variety of recording media such as paper having a luxurious, leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording media have a coarse surface acquired through embossing to produce that luxurious impression. When transferring the toner image onto such recording media, toner does not transfer well to recessed portions of the surface as compared with projecting portions on the surface. As a result, the toner image is not transferred well to the recessed portions of the surface, and an image density at the recessed portions is lower than the image density at the projecting portions, which appears as a pattern of light and dark patches on a resulting output image.
In order to prevent inadequate transfer of toner in the recessed portions of the recording medium surface, in one approach, a transfer bias (hereinafter referred to as a superimposed transfer bias), in which an alternating current (AC) component is superimposed on a DC component and the polarity changes with time, is used. Such a configuration is proposed in JP-2012-63746-A. In this configuration, the superimposed transfer bias causes the toner to move back-and-forth between the recessed portions of the surface of the recording medium and the image bearing member, thereby moving the toner to the recessed portions.
Although advantageous, when using the superimposed transfer bias as a transfer bias to transfer the toner image from the image bearing member to the recording medium, improper transfer of toner such as white spots also known as dropouts occurs easily in the image on the recording medium. The white spots are generated when electrical discharge occurs in a transfer nip at which the image bearing member and the intermediate transfer belt meet and press against each other and the toner at the place where the electrical discharge occurs loses its charge. As a result, the toner fails to be transferred to the recording medium.
In view of the above, if the electrical discharge is prevented by reducing the maximum potential difference between the image bearing member and the recording medium in the transfer nip, formation of the white spots may be suppressed.
However, the purpose of applying the superimposed transfer bias as a transfer bias lies in moving the toner back-and-forth between the image bearing member and the recording medium in the transfer nip so that the toner is reliably transferred to recessed portions of the recording medium. In order to achieve such movement of toner, a significant amount of voltage is required as a peak-to-peak voltage of the superimposed transfer bias. For this reason, the peak-to-peak voltage of the superimposed transfer bias cannot be reduced too much, and hence the maximum potential difference between the image bearing member and the recording medium in the transfer nip cannot be reduced adequately. Consequently, white spots are easily generated, when using the superimposed transfer bias.
The similar problem occurs when an image is formed on a recording medium having a relatively smooth surface using the superimposed transfer bias as a transfer bias.